Phase Transformation Originated From DiffusionInduced Grain Boundary Migration

Superlattice phase transformation, occurred in the solute enriched region caused by diffusion induced grain boundary migration (DIGM), was found in two kinds of diffusion couples which consist of: (1) low carbon high strength steel with hot dipped zinc coating, and (2) commercial pure iron with galvanized zinc coating after isothermally diffusion annealed at different temperatures (500, 560 and 580℃) with various durations. The product of the transformation is an ordered phase with Li₂ superlattice structure which was identified through the analysis by SEM,TEM and AEM. The ordered phase resulted from transformation occurred in the alloyed areas generated by DIGM and DIR The zinc content in this phase is variable, it approaches the stoichiometric composition Fe₃Zn as the diffusion time is sufficiently prolonged. The mechanism of the transformation in connection with the process of DIGM and DIR was discussed. DIGM DIR and DIP (diffusion induced phase transformation) are three interrelated processed originated from grain boundary diffusion.